The present invention generally relates to sealants for ceramics and, more particularly, to a composite sealant for solid oxide fuel cells that provides improved thermal matching and gap holding capacity.
A fuel cell is basically a galvanic conversion device that electrochemically reacts a fuel with an oxidant within catalytic confines to generate a direct current. A fuel cell typically includes a cathode material which defines a passageway for the oxidant and an anode material which defines a passageway for the fuel. An electrolyte is sandwiched between and separates the cathode and anode materials.
The fuel and oxidant fluids are usually gases and are continuously passed through separate cell passageways. Electrochemical conversion occurs at or near the three-phase boundary of the electrodes (cathode and anode) and electrolyte. The fuel is electrochemically reacted with the oxidant to produce a DC electrical output. The anode or fuel electrode enhances the rate at which electrochemical reactions occur on the fuel side. The cathode or oxidant electrode functions similarly on the oxidant side.
Specifically, in a solid oxide fuel cell (SOFC), the fuel reacts with oxide ions on the anode to produce electrons and water, the latter of which is removed in the fuel flow stream. The oxygen reacts with the electrons on the cathode surface to form oxide ions that diffuse through the electrolyte to the anode. The electrons flow from the anode through an external circuit and then to the cathode, with the circuit being closed internally by the transport of oxide ions through the electrolyte.
In a SOFC, the electrolyte is in a solid form. Typically, the electrolyte is made of a nonmetallic ceramic, such as dense yttria-stabilized zirconia (YSZ) ceramic, that is a nonconductor of electrons which ensures that the electrons must pass through the external circuit to do useful work. As such, the electrolyte provides a voltage buildup on opposite sides of the electrolyte, while isolating the fuel and oxidant gases from one another. The anode and cathode are generally porous, with the anode oftentimes being made of nickel/YSZ cermet and the cathode oftentimes being made of doped lanthanum manganite. In the solid oxide fuel cell, hydrogen or a hydrocarbon is commonly used as the fuel, while oxygen or air is used as the oxidant.
An individual SOFC cell usually generates a relatively small voltage. Thus, to achieve higher voltages that are useful, the individual electrochemical cells are connected together in series to form a stack. Electrical connection between cells is achieved by the use of an electrical interconnect between the cathode and anode of adjacent cells. Also typically included in the stack are ducts or manifolding to conduct the fuel and oxidant into and out of the stack.
Whether in the form of a stack or individual cells, it is important that the fuel and oxidant be kept separate from one another. Otherwise, there would be less efficiency in producing the exchange of ions across the electrolyte, as well as a potential for explosions. Consequently, sealants have been used to seal different portions of a SOFC stack, such as at the edges and where manifolds must be affixed to the stack.
Generally, the sealant should exhibit one or more of the following characteristics. The sealant has a relatively high coefficient of thermal expansion (CTE) to match that of the stack components, including the cells, interconnects, and manifold materials. Typically, those components have CTE's ranging from about 9 to 15.times.10.sup.-6 /.degree. C. The sealant also has a desirable viscosity such that the sealant is fluid enough to seal gaps at the sealing/assembly temperature and be viscous enough at the cell operating temperature (about 700-1000.degree. C. for an SOFC) so that gaps are kept sealed under gas pressure differentials. Also during fuel cell operating conditions and environments, the sealant should be stable both thermally (i.e., negligible crystallization) and chemically (i.e., negligible weight loss, minimum reaction with stack and manifold materials).
Various attempts have been made to achieve the above sealant characteristics, in addition to others. For example, in U.S. Pat. No. 4,774,154, a sealant was provided for a fuel cell that operated at about 400.degree. F. The sealant comprised an elastomer, such as a flourinated hydrocarbon. Added to the elastomer was a reinforcing filler to improve processing and molding characteristics, such as carbon black. A blowing agent (such as azodicarbonamide), blowing agent promoter, and acid acceptor (to absorb acid during curing) were additionally provided in the sealant. However, such sealant would have little, if any, utility in the context of an SOFC which operates at temperatures much higher than 400.degree. F.
Likewise, the sealant in U.S. Pat. No. 5,110,691 was particularly useful for fuel cells operating at about 140 to 250.degree. F. The sealant comprised butyl rubber and ethylene propylene. A filler was added to extend sealant life, as well as improve creep and strength properties. An anti-oxidant was used in the sealant to improve stability, and a thickener was used to improve viscosity. Again, however, such sealant would evidently not be useful at operating temperatures well in excess of 250.degree. F.
A sealant claimed to be useful for an SOFC is found in U.S. Pat. No. 5,453,331. Therein, it was claimed that commonly used silica based sealants had higher softening temperatures than the boron oxide based sealant of the invention. It was also believed that a low silica content avoided formation of volatile silicon monoxide in the anode side of the cell. Accordingly, the silica content was taught to be kept between about 3 to 20 mol %. Alumina was added to retard crystallization and lanthanum oxide was added to modify the viscosity. A curing agent was also provided in the sealant to improve strength. While apparently providing advantages, it can be seen that multiple steps and multiple reactants are required. Also, the two main ingredients of the sealant, SrO and La.sub.2 O.sub.3, are relatively expensive materials, which increases the cost of the sealant. Further, there seems to be no suggestion that the sealant is capable of providing a seal to relatively large gaps on the order of about &gt;1 mm.
As can be seen, there is a need for an improved sealant, including ones for ceramics and solid oxide fuel cells. Also needed is a sealant that can be used at operating temperatures of about 700-1000.degree. C. A sealant is needed that can provide improved gap holding capacity for gaps greater than about 1 mm. A further need is for a low cost sealant that is relatively easy to manufacture as a result of requiring fewer components and less reaction steps.